a person, not necessarily a family member of any kind, who, when things around them are generally well will be ecstatic about any and everything, but similarly will turn dour/morose/angry/violent the moment things go slightly awry.

Dude1: Barry, is such a fucking drunk uncle. I can't stand it when he flips out like that over nothing.
Dude2: Uh... Barry isn't your uncle, and I'm pretty sure he hasn't been drunk the last two times we saw him.
Dude1: ...You're an idiot...